Such an exhaust muffler is known from EP 1 055 804 B1, for example, and has a housing through which a pipe passes. This pipe is designed so that it is permeable for airborne sound in a first space due to the fact that it has a perforated pipe section in the first space. In addition, the known exhaust muffler has an antisound generator in the form of a loudspeaker arranged in a second space. During operation of the exhaust muffler, the antisound generator generates antisound to act on a third space through a wall opening. Said wall opening is provided in a partition separating the second space from the third space. In addition, the first space communicates with the third space through a sound outlet for transmission of airborne sound.
Additional active exhaust mufflers are described in EP 0 373 188 B1, U.S. Pat. No. 5,233,137, U.S. Pat. No. 4,177,874, U.S. Pat. No. 5,229,556, U.S. Pat. No. 5,336,856, U.S. Pat. No. 5,319,165, U.S. Pat. No. 5,432,857, EP 0 674 097 A1, U.S. Pat. No. 5,619,020, U.S. Pat. No. 5,600,106, EP 0 916 817 and DE 197 51 596.
During operation of such an active exhaust muffler, the airborne sound to be suppressed is output from the pipe in the first space and is introduced through the sound outlet into the third space. At the same time, antisound, i.e., sound with a reciprocal pressure level characteristic in comparison with that of the sound to be absorbed, is generated and supplied to the third space with the help of the loudspeaker. Then in the third space there is a mutual extinction of sound and antisound, thus characterizing an active exhaust muffler of the present type.
In theory, effective noise suppression can be achieved with the help of such an active exhaust muffler. In practice, however, serious problems occur because of the high temperatures prevailing in the exhaust and because the loudspeakers available in the past have not had a long enough lifetime at such high operating temperatures. Furthermore, the loudspeakers required in active exhaust mufflers must be very powerful to be able to respond appropriately to the extremely high sound pressure levels prevailing in the exhaust and to be able to achieve the desired noise suppression effect. However, a high power level leads to a high additional heat production in the loudspeaker.